The invention relates to a hot-rolled steel with very high elasticity limit and mechanical resistance usable in particular for auto parts production.
In the field of hot-rolled steel sheet production, a steel whose characteristics are obtained by controlled rolling, products with high elasticity limits is known, i.e., 315 MPa to 700 MPa.
In the field of hot-rolled metal sheets produced from a strip train, the operating performance of parts obtained by molding from these sheets is an important criterion because it defines the useful life of the molded parts, for example, by stamping, profiling or hydroforming.
Operating performance in connection with performance with fatigue defines the useful life for specific charges.
To improve the fatigue resistance of molded parts, one solution consists of using very high-resistance steels with considerable fatigue resistance properties, because at first glance, there is a proportional relationship between maximum endurance and mechanical resistance. Nevertheless, the steel must be able to be stamped. In general, molding properties decrease as mechanical resistance increases, thereby limiting the molding possibilities for parts able to be manufactured from high-resistance steel.
Shock-resistance is also an important characteristic for reasons of safety, namely in automobile applications, since shock-resistance defines the resistance to sudden breakage of the parts. To improve this characteristic of molded parts, a solution consists of using very high-resistance steels with considerable fatigue resistance properties, because at first glance, there is a linear relationship between shock-resistance and maximum elasticity. However, molding properties generally decrease as maximum elasticity increases.
In the range of common flat hot-rolled products, the mechanical characteristics of which are obtained by controlled rolling on a wide-strip train, in particular there are four primary categories of steel with superior mechanical characteristics.
HLE steels with high maximum elasticity are micro-alloy steels with maximum elasticity ranging from 315 MPa to 700 MPa, but having limited moldability in particular because of a high Re/Rm ratio greater than 0.85. These steels have a carbureted ferrite-phase structure of the cementite kinds. The elasticity maximum is obtained by controlled rolling and precipitation of the micro-alloying elements such as niobium, vanadium and titanium during ferrite transformation.
Dual-phase steels are steels with a ferrite martensite structure with noteworthy molding properties. Mechanical resistance levels generally range from 550 MPa to 800 MPa. The highest level is obtained by precipitation of micro-alloying elements during the ferrite transformation that completes the hardening effect of martensite.
HR steels are steels referred to as high-resistance, with carbon and manganese and undergoing relatively rapid cooling after rolling, along with low-temperature coiling, to give them a ferrite-baintic structure. Their intermediate moldability level is between that HLE steels and that of dual-phase steels. Resistance levels range from 450 MPa to 800 MPa.
Martensite steels have the highest resistance levels. These steels have a martensite structure obtained by heat treatment after rolling. It is difficult to produce this kind of structure on a wide-strip train because of the fragility of martensite, which causes the strip to break after rolling. Martensite steels make it possible to achieve resistance levels above 1,000 MPa but with very low ductility levels and expansions of less than 8%. In addition, a heat treatment must be carried out after rolling.
Increasing the resistance level of all of the above-mentioned steels entails an increase in rolling efforts, thereby limiting the reduction in thickness and not allowing the full benefits of alloying.
The goal of the invention is to present a hot-rolled steel with very high maximum elasticity mechanical resistance and good molding characteristics to produce parts by stamping, profiling and hydroforming, namely for the auto industry.
The object of the invention is a hot-rolled steel with very high maximum elasticity and mechanical resistance usable in particular for producing auto parts, characterized by the following composition by weight:
0.08% less than carbon less than 0.2%
1% less than manganese less than 2%
0.02% less than aluminum less than 0.1%
silicon less than 0.5%
phosphorus less than 0.03%
sulfur less than 0.01%
vanadium less than 0.3%
chromium less than 1%
nitrogen less than 0.015%
molybdenum less than 0.6%
the rest being of steel and impurities inherent in processing.
The steel is preferably characterized by the following composition by weight:
0.1% less than carbon less than 0.14%
1.4% less than manganese less than 1.8%
0.02% less than aluminum less than 0.08%
0.15% less than silicon less than 0.3%
phosphorus less than 0.03%
sulfur less than 0.008%
0.1% less than vanadium less than 0.3%
0.3% less than chromium less than 0.6% nitrogen less than 0.012%
0.15 less than molybdenum less than 0.4
the rest being of iron and impurities inherent in processing. The invention also relates to a process for producing a hot-rolled steel sheet strip with very high resistance usable in particular to produce auto parts and characterized in that the steel has the following composition by weight:
0.08% less than carbon less than 0.2%
1% less than manganese less than 2%
0.02% less than aluminum less than 0.1%
silicon less than 0.5%
phosphorus less than 0.03%
sulfur less than 0.01%
vanadium less than 0.3%
chromium less than 1%
nitrogen less than 0.015%
molybdenum less than 0.6%
the rest being of iron and impurities inherent in processing is subjected to:
rolling at a temperature below 950xc2x0 C. and preferably below 880xc2x0 C.,
cooling carried out at a rate of more than 20xc2x0 C. per second and preferably at a rate ranging from 100xc2x0 C. to 200xc2x0 C. per second up to a temperature ranging from 400xc2x0 C. to 600xc2x0 C., and preferably up to a temperature ranging from 450xc2x0 C. to 500xc2x0 C.